1. Field of the Invention
The present invention relates to image-signal processing apparatuses suited to digital video equipment, and more particularly, to an image-signal processing apparatus and method which always form the best still-image signal from a video signal generated by either a so-called interlacing image method or a so-called progressive image method.
2. Description of the Related Art
In conventional video units, video signals of the interlacing image method are used for capturing and recording, which include first and second fields of an image in which, generally, adjacent fields have different time axes. Since video signals generated by the interlacing image method during capturing and recording have different time axes in alternate fields, motion images having smooth movement are obtained. Therefore, capturing and recording are usually performed by the interlacing image method in digital video units.
There is a demand for such video units which can generate a still-image signal from a recorded motion-image video signal and display it. However, when a one-frame still image is generated from a video signal formed, for example, by the above-described interlacing image method, since first and second fields have different time axes, a flicker is generated when there is a portion which has movement between the fields and image quality deteriorates. To eliminate this problem, such a portion having movement is detected, and the image shown at the portion is modified to remove the flicker.
Specifically, in FIG. 2, in a conventional apparatus, a digital-image data stream formed, for example, by the interlacing image method, is input to an input terminal 1. The data stream input via the input terminal 1 is sent to a decoder circuit 2 which converts data streams to video signals to generate digital image signals including first and second fields which conform, for example, to the standard TV signal system and in which respective images are formed of alternate horizontal scanning lines in frames. The decoder circuit 2 outputs a digital video signal including these digital image signals.
The image signal in each field is sent from the decoder circuit 2 to a field memory 3, and an image signal taken out from the field memory 3 is sent to a field memory 4. The field memories 3 and 4 receive writing control signals applied to terminals 5 and 6, and reading control signals applied to terminals 7 and 8. These signals control writing signals into and reading signals from the field memories 3 and 4. The digital image signals in fields of the digital video signal output from the decoder circuit 2 are sequentially written into the field memories 3 and 4.
Images read from the field memories 3 and 4 are sent to a motion detection filter 9, and portions having movement which are likely to cause flickering between fields are detected. A detection signal is sent to an image interpolation filter 10. In addition, the images read from the field memories 3 and 4 are sent to the interpolation filter 10 through delay circuits 11 and 12 which correspond to the detecting filter 9. With these operation, the interpolation filter 10 modifies the image shown at the portion having the detected movement and read from the field memory 3 by using the image read from the field memory 4 so as to eliminate flickering.
With such modification, the sharpness of the portion where modification has been applied is reduced, and the difference in image quality between the portion and other portions where modification has not been applied becomes conspicuous. Therefore, filter processing for reducing sharpness to the same extent as that for the portion where modification has been applied is performed for the portions where modification has not been applied to make the entire image quality uniform. Specifically, the image interpolation filter 10 performs filter processing for the entire signal so as to make the entire image quality uniform. A filter 13 also applies the same processing to an image signal output from the delay circuit 11.
A selector 14 selects alternately, in fields, an image signal sent from the image interpolation filter 10 and an image signal sent from the filter 13 to output from an output terminal 15 a still-image signal including a first field, and a second field to which modification has been applied so as not to generate a flicker. A motion-image signal sent from the delay circuit 11 is taken out from a delay circuit 16 corresponding to the image interpolation filter 10 and the filter 13, and the motion-image signal is selected by a selector 17 and is output from the output terminal 15 through the selector 14.
Selections at the above-described selectors 14 and 17 are made according to a still-image/motion-image mode signal input via a control terminal 18, and a first/second field selection signal input to a control terminal 19. When the mode signal indicates a motion image, namely, is “1,” this mode signal “1” is inverted by an inverter circuit 20 and the inverted signal “0” is input to the selector 17 to select the delay-circuit-16 side (B). In addition, the mode signal “1” is input to the selector 14 through an OR circuit 21 to select the selector-17 side (A), so that a motion-image signal is output from the output terminal 15.
When the mode signal indicates a still image, namely, is “0,” this mode signal “0” is inverted by the inverter circuit 20 and the inverted signal “1” is input to the selector 17 to select the filter-13 side (A). In addition, the first/second field selection signal is input through the OR circuit 21 to the selector 14 to alternately select, in fields, the image signal (B) sent from the image interpolation filter 10 and the image signal (A) sent from the filter 13 and selected by the selector 17, so that a still-image signal having first and second fields is output from the output terminal 15.
Therefore, according to the above-described apparatus, when the mode signal indicates a motion image, a video signal generated by the interlacing image method is taken out as a motion-image signal. When the mode signal indicates a still image, a still-image signal having a first field, and a second field to which modification has been applied so as not to cause flickering, is taken out. With these operations, from a video signal generated by the interlacing image method, when the mode signal indicates a motion image, the original motion-image signal is obtained as is, and when the mode signal indicates a still image, an acceptable still-image signal having no flicker is obtained.
In some camcorder apparatuses, which include a digital video cassette recorder (VCR), for example, capturing and recording are performed by the interlacing image method, in which a video signal has first and second fields of an image having different time axes in alternate fields, and in addition, as required, capturing and recording are also performed by the progressive image method, in which a video signal has first and second fields obtained by extracting every other horizontal scanning line from images having different time axes in alternate frames.
Since capturing and recording are performed by both the interlacing image method and the progressive image method as described above, a video signal generated by the interlacing image method during capturing or recording has different time axes in alternate fields, and therefore, a motion image having a smooth movement is obtained; and a video signal generated by the progressive image method during capturing or recording has the same time axis in one frame, and therefore, a high-resolution image is obtained. When a still image is formed by this method, a higher-quality still image signal is obtained.
When a motion image and a still image are switched in such a way by the use of the apparatus shown in FIG. 2, even if a video signal generated by the progressive image method is input, a signal is obtained through the image interpolation filter 10. When a video signal generated by the progressive image method is input, since a detection is not made by the detecting filter 9, interpolation processing is not performed. But, filter processing, which makes the entire image quality uniform, is applied to the entire signal. The same processing is also applied to an image signal sent from the delay circuit 11 by the filter 13.
Since video signals generated by the progressive image method during capturing and recording have the same time axis in one frame, flickering does not occur. Therefore, it is neither necessary to modify an image to eliminate flickering nor to perform filter processing to reduce sharpness. Consequently, it is relatively easy to generate still images having a high sharpness with just the use of one frame memory.
In conventional digital video units which can perform capturing and recording by the progressive image method and by the interlacing image method, since circuits for these methods are shared, the same processing is applied to video signals of the progressive image method and those of the interlacing image method. Therefore, even when a video signal of the progressive image method is input, the same filter processing as that performed when a video signal of the interlacing image method is input is applied, and the obtained image has the same image quality as a conventional still image generated from a video signal of the interlacing image method.
To prevent sharpness from being reduced, it can be considered that the precision of motion detection is improved. For example, motion is detected in units of pixels to prevent sharpness from being reduced in images generated by both the progressive image method and the interlacing image method. To implement this idea, however, the size of circuits such as a memory and gate elements used for motion detection needs to be increased. It is very difficult to provide such circuits, for example, for camcorder apparatuses, for example, which are required to be compact and light-weight.